Fix: cross-version placeholder support (#3463)

source/__.js

@@ -1,4 +1,3 @@
-import _placeholder from './internal/_placeholder.js';
 /**
  * A special placeholder value used to specify "gaps" within curried functions,
  * allowing partial application of any combination of arguments, regardless of
@@ -26,4 +25,5 @@ import _placeholder from './internal/_placeholder.js';
  *      const greet = R.replace('{name}', R.__, 'Hello, {name}!');
  *      greet('Alice'); //=> 'Hello, Alice!'
  */
-export default _placeholder;
+export default {'@@functional/placeholder': true};
+

source/internal/_isPlaceholder.js

@@ -1,5 +1,5 @@
-import _placeholder from './_placeholder.js';
-
 export default function _isPlaceholder(a) {
-  return a === _placeholder;
+  return a != null &&
+         typeof a === 'object' &&
+         a['@@functional/placeholder'] === true;
 }